Add Buffer "constructor-kwargs" header

Part of `Buffer`'s serializers' work includes handling subclasses (like
cuML's `Array`). However it doesn't handle any keyword arguments those
subclass constructors may need. This changes that by adding an optional
(by default trivial) "constructor-kwargs" header. Subclasses can then
call their parents' serializer and pack this header with relevant
content afterwards. This way during deserialization this content can be
passed into the subclass' constructor.

CHANGELOG.md

@@ -14,6 +14,7 @@
 - PR #4053 Multi-column quantiles.
 - PR #4107 Add groupby nunique aggregation
 - PR #4153 Support Dask serialization protocol on cuDF objects
+- PR #4164 Add Buffer "constructor-kwargs" header
 
 ## Improvements
 

python/cudf/cudf/core/buffer.py

@@ -106,13 +106,14 @@ def _init_from_array_like(self, data):
     def serialize(self):
         header = {}
         header["type-serialized"] = pickle.dumps(type(self))
+        header["constructor-kwargs"] = {}
         header["desc"] = self.__cuda_array_interface__.copy()
         frames = [self]
         return header, frames
 
     @classmethod
     def deserialize(cls, header, frames):
-        buf = cls(frames[0])
+        buf = cls(frames[0], **header["constructor-kwargs"])
 
         if header["desc"]["shape"] != buf.__cuda_array_interface__["shape"]:
             raise ValueError(